1. Field of the Invention
The present invention relates to a base station for a radio system, the base station supervising the quality of a radio link between the base station and mobile stations, and includes a generator circuit for generating a supervising signal having a desired baseband frequency; a measuring circuit for monitoring the level of the baseband supervising signal; a transmitting circuit for frequency-modulating a transmitting-frequency carrier of the baseband supervising signal and transmitting it to the mobile radio station, a receiving circuit for detecting the supervising signal transmitted back from the mobile radio station and modulating it to a receiving frequency; and a signal processing circuit for determining the quality of the used radio link.
2. Description of the Related Art
In the NMT (Nordic Mobile Telephone) mobile telephone system, the quality of the radio link between a base station and a mobile telephone is supervised by special supervising signals. The base station generates a supervising signal, e.g. by lowpass-filtering from a clock signal having a desired supervising signal frequency. The NMT system utilizes four supervising signal frequencies with spacings of 30 Hz. The base station transmits a supervising signal to the mobile telephone, which returns the supervising signal immediately to the base station, and the base station determines the quality of the used radio link, especially the signal-to-noise (S/N) ratio, by way of the received supervising signal.
When the level of the baseband supervising signal generated at the base station changes for some reason before transmission, this change in level appears as a number of deviation variations in the radio-frequency signal transmitted from the base station to the mobile telephone and frequency-modulated by the supervising signal and also affects the measuring accuracy of the S/N ratio of the radio link. In order to ensure that the measured S/N ratio describes the quality of the radio link sufficiently accurately, the NMT specifications define that the deviation caused by the supervising signal must not deviate from a set value by more than 10%. Today the level of the supervising signal is monitored by a measuring circuit comprising a rectifier and comparators, for which appropriate threshold voltages corresponding to the lowest and the highest allowable level of the supervising signal are set by a calibration circuit including adjustable resistors. Such a measuring circuit is, however, slow and difficult to calibrate, as it requires at least two adjusters and four adjusting procedures.